U.S. patent application number 12/519767 was filed with the patent office on 2010-02-18 for qos sceduling methods for wlans with heterogenous applications.
This patent application is currently assigned to KONINKLIJKE PHILIPS ELECTRONICS N. V.. Invention is credited to Dave Cavalcanti, Ruediger Schmitt.
Application Number | 20100039973 12/519767 |
Document ID | / |
Family ID | 39427640 |
Filed Date | 2010-02-18 |
United States Patent
Application |
20100039973 |
Kind Code |
A1 |
Cavalcanti; Dave ; et
al. |
February 18, 2010 |
QOS SCEDULING METHODS FOR WLANS WITH HETEROGENOUS APPLICATIONS
Abstract
A wireless system (100) includes at least one power-save
wireless station (PS STA)(102). A method of wireless includes
allocating deterministic time intervals (206,207) to the
PS-STAs.
Inventors: |
Cavalcanti; Dave; (Ossining,
NY) ; Schmitt; Ruediger; (Maplewood, NJ) |
Correspondence
Address: |
PHILIPS INTELLECTUAL PROPERTY & STANDARDS
P. O. Box 3001
BRIARCLIFF MANOR
NY
10510
US
|
Assignee: |
KONINKLIJKE PHILIPS ELECTRONICS N.
V.
Eindhoven
NL
|
Family ID: |
39427640 |
Appl. No.: |
12/519767 |
Filed: |
December 4, 2007 |
PCT Filed: |
December 4, 2007 |
PCT NO: |
PCT/IB07/54921 |
371 Date: |
June 18, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60870429 |
Dec 18, 2006 |
|
|
|
Current U.S.
Class: |
370/311 ;
370/329 |
Current CPC
Class: |
Y02D 30/70 20200801;
Y02D 70/144 20180101; H04W 52/0216 20130101; H04W 74/04 20130101;
Y02D 70/142 20180101; Y02D 70/23 20180101; H04W 84/12 20130101;
Y02D 70/146 20180101 |
Class at
Publication: |
370/311 ;
370/329 |
International
Class: |
H04W 72/12 20090101
H04W072/12; H04W 52/02 20090101 H04W052/02 |
Claims
1. A wireless system, comprising: a plurality of wireless stations
(STA), wherein at least one of the STAs is a power save (PS) STA;
another wireless STA (101) adapted to control access to a medium of
a wireless network, wherein the at least one PS STA is provided
deterministic access to the medium before other STAs of the
plurality of STAs.
2. A wireless system as claimed in claim 1, wherein the another
wireless STA is an access point (AP) and the wireless network is a
centralized wireless network.
3. A wireless system as claimed in claim 2, wherein the AP
transmits a beacon to the STAs at the beginning of a superframe or
a beacon interval.
4. A wireless system as claimed in claim 3 wherein a power-save
(PS) communication period succeeds a beacon transmission period
(BT) and precedes other communication periods.
5. A wireless system as claimed in claim 4, wherein the
deterministic access is provided by a assigning power save
transmission opportunity (PS-TXOP) to each of the at least one PS
STA in the PS communication period.
6. A wireless system as claimed in claim 4, wherein a PS-TXOP is
granted to each of the at least one PS STAs at a respective
deterministic start time and consecutive PS-TXOPs are granted to
the same PS STA at deterministic start times in consecutive PS
periods.
7. A wireless system as claimed in claim 1, wherein the at least
one PS STA comprises a medical device.
8. A wireless system as claimed in claim 6, wherein the medical
device is mobile or portable having constrained power
resources.
9. A wireless system as claimed in claim 5, wherein each of the at
least one PS STAs is adapted to enter a sleep mode after
transmitting or receiving data.
8. A wireless system as claimed in claim 2, wherein the AP
maintains a list of each of the at least one PS STAs in the
system.
9. A wireless system as claimed in claim 2, wherein the AP
maintains a list of STAs, which are not PS STAs in the system.
10. A method of wireless communication, the method comprising:
scheduling a power save transmission opportunity (PS-TXOP) in a
power save (PS) period after a termination of a beacon transmission
period (BT) and before other transmission opportunities
(TXOPs).
11. A method as claimed in claim 10, further comprising
transmitting data from a PS STA to another STA during an assigned
PS-TXOP, wherein, after the transmitting, the PS-STA enters a sleep
mode.
12. A method as claimed in claim 10, further comprising
transmitting data from a PS-STA to another STA during an assigned
PS-TXOP, wherein, after the transmitting, but before an end of the
PS-TXOP, allocating a remaining portion of the PS-TXOP to another
STA.
13. A method as claimed in claim 12, wherein the STA is an access
point (AP) and the another STA is not a PS STA.
14. A method as claimed in claim 12, wherein the another STA is not
a PS STA.
15. A method as claimed in claim 12, wherein the another STA is a
real time multimedia (RT) STA.
16. A method as claimed in claim 10, further comprising, before the
scheduling, maintaining a list of a power save (PS) STA.
17. A method as claimed in claim 10, further comprising, before the
scheduling, maintaining a list of a STAs, which are not (PS)
STAs.
18. A method as claimed in claim 10, further comprising: scheduling
consecutive PS-TXOPs for a PS STA at deterministic start times.
19. A method as claimed in claim 10, wherein the consecutive
PS-TXOPs are granted to the same PS STA at deterministic start
times in consecutive power save (PS) communication periods.
20. A method as claimed in claim 10, further comprising
transmitting data from the AP or other STA to the PS STA within the
PS-TXOP, and after receiving an acknowledgement (ACK) from the PS
STA, the AP or other STA allocates a remaining portion of the
PS-TXOP to another STA.
Description
[0001] Wireless communication technology has significantly advanced
making the wireless medium a viable alternative to wired solutions.
As such, the use of wireless connectivity in data and voice
communications continues to increase. These devices include mobile
telephones, portable computers in wireless networks (e.g., wireless
local area networks (WLANS), stationary computers in wireless
networks, portable handsets, to name only a few).
[0002] An ever-present challenge in standards such as IEEE 802.11
and its progeny is to provide Quality of Service (QoS) guarantees
to real-time and critical applications, such as voice and medical
applications, while minimizing the power consumed by certain mobile
wireless stations (STAs). Recently, the IEEE 802.11e amendment to
the IEEE 802.11 standard has been approved. IEEE 802.11e defines
the Hybrid Coordination Function (HCF) Controlled Channel Access
(HCCA) operation mode to support parameterized QoS through a Hybrid
Coordinator (HC), which is an access point (AP) that controls the
access to the medium and grants transmission opportunities (TXOPs)
to the STAs according to a centralized transmission schedule.
[0003] In known systems, APs control access to the medium by, among
other tasks, scheduling access to the medium. In such known
systems, the AP schedules wireless stations (STAs) with the lowest
delay requirements (earlier deadlines) first. In this way, the
reliability of STAs with low delay requirements is improved by
scheduling these STAs with greater priority in the data transfer
period. By contrast, STAs with larger delay requirements (later
deadlines) may not be able to access the medium and, as such, may
receive a lower quality of service compared to STAs with lower
delay requirements. By way of example, a voice application, which
has a low delay requirement, typically less than approximately 20
ms, is granted priority over a medical STA that normally can
support comparatively higher delays, which can be as great as
approximately 250 ms.
[0004] Moreover, known scheduling algorithms fail to account for
reliability requirements (e.g., packet loss requirements) and power
consumption requirements of STAs. As will be appreciated, in
certain applications, reliability of service is exceedingly
important; while in other applications it is useful, if not
essential for STAs to conserve electrical power to ensure
longevity.
[0005] There is a need, therefore, for a method and system that
overcomes at least the shortcomings described above.
[0006] In accordance with an illustrative embodiment, a wireless
system includes a plurality of wireless stations (STAs) and at
least one of the STAs is a Power-Save (PS) STA. The system also
includes another wireless STA adapted to provide access to a medium
of a wireless network. The PS STA is provided deterministic access
to the medium before other STAs of the plurality of STAs.
[0007] In accordance with yet another illustrative embodiment, a
method of wireless communication includes scheduling a power save
transmission opportunity (PS-TXOP) in a power save (PS) period
after a termination of a beacon transmission and before other
transmission opportunities (TXOPs).
[0008] The invention is best understood from the following detailed
description when read with the accompanying drawing figures. It is
emphasized that the various features are not necessarily drawn to
scale. In fact, the dimensions may be arbitrarily increased or
decreased for clarity of discussion. Wherever practical, like
reference numerals refer to like elements in the drawing
figures.
[0009] FIG. 1 is a simplified schematic diagram of a wireless
communication system in accordance with a representative
embodiment.
[0010] FIG. 2 is a timing diagram of data communication in
accordance with a representative embodiment.
[0011] FIG. 3 is a timing diagram of data communication in
accordance with a representative embodiment.
[0012] FIG. 4 is a flow chart of a method of wireless communication
in accordance with a representative embodiment.
[0013] FIG. 5 is a flow chart of a method of wireless communication
in accordance with a representative embodiment.
[0014] As used herein, the terms `a` and `an` mean one or more; and
the term `plurality` means two or more.
[0015] As used herein, the term `deterministic` means having a time
evolution that can be predicted with substantial precision. For
example, a deterministic time interval is a time interval that has
a beginning and an end that can be predicted with substantial
precision.
[0016] In the following detailed description, for purposes of
explanation and not limitation, representative embodiments
disclosing specific details are set forth in order to provide a
thorough understanding of the present teachings. However, it will
be apparent to one having ordinary skill in the art having had the
benefit of the present disclosure that other embodiments that
depart from the specific details disclosed herein. Moreover,
descriptions of well-known devices, methods, systems and protocols
may be omitted so as to not obscure the description of the
representative embodiments. Nonetheless, such devices, methods,
systems and protocols that are within the purview of one of
ordinary skill in the art may be used in accordance with the
representative embodiments. Finally, wherever practical, like
reference numerals refer to like features.
[0017] It is noted that in the illustrative embodiments described
herein, the network may be a wireless network with a centralized
architecture or a decentralized architecture. Illustratively, the
network may be one which functions under a DSA Medium Access (MAC)
layer, such as to be defined under IEEE 802.22, or as defined under
the ECMA 368 standard, IEEE 802.16, IEEE 802.11, or IEEE 802.15.
The disclosures of the referenced specifications are specifically
incorporated herein by reference in their entirety.
[0018] Moreover, the network may be a cellular network; a wireless
local area network (WLAN); a wireless personal area network (WPAN);
a wireless body area network (WBAN) or a wireless regional area
network (WRAN). Furthermore, the MAC protocol may be a time
division multiple access (TDMA) protocol; a carrier sense multiple
access (CSMA) protocol; a CSMA with collision avoidance (CSMA/CA)
protocol; a Code Division Multiple Access (CDMA) protocol; or a
frequency division multiple access (FDMA) protocol. It is
emphasized that the noted networks and protocols are merely
illustrative and that networks and protocols other than those
specifically mentioned may be used without departing from the
present teachings.
[0019] FIG. 1 is a simplified schematic view of a wireless system
100 in accordance with an illustrative embodiment. The wireless
system 100 may comprise a centralized network and include an access
point (AP) 101, which is also referred to as a base station (BS),
or as an HC. The wireless system 100 further comprises a plurality
of wireless stations, which also may be referred to as wireless
stations (STAs) or Customer Premise Equipment (CPE).
[0020] While the description that follows relates primarily to a
centralized network with AP 101, distributed networks are clearly
contemplated by the present teachings. As will be readily
appreciated by one of ordinary skill in the art, in a distributed
system, the AP 101 is not provided. Rather, another STA(s) controls
access to the medium, and fulfills the functionality of the AP
101.
[0021] Illustratively, the wireless system 100 may comprise one of
the types of networks noted previously. Moreover, the STAs may be
computers, mobile telephones, personal digital assistants (PDA),
wireless sensors, or similar device that typically operates in such
networks. In a specific embodiment, at least one of the STAs is
stationary. It is also contemplated that the STAs may be adapted to
function in restricted frequency channels of a frequency band that
requires protection of incumbent users or in frequency channels of
an unlicensed frequency band. Often, in the interest of simplicity,
restricted frequency channels, restricted channels and frequency
channels in unlicensed frequency bands may be referred to herein as
simply `channels.`
[0022] The system 100 includes low-power (also referred to as
power-save) STAs (PS STAs) 102 and real time multi-media STAs (RT
STAs) 103. In a representative embodiment, the STAs 102 are adapted
to enter a power-save (PS) mode, during which the STAs 102 are
essentially `asleep`, which is a common term of art meaning the STA
is in a mode of not receiving or transmitting data (and thereby
conserving power).
[0023] Generally, PS STAs 102 could be any wireless device that has
constrained power resources or that aims to minimize the consumed
power to access the network. During PS mode the STAs 102 may be
performing certain functions that normally do not require
significant power resources. As such STAs and PS mode are known to
one skilled in the art, details are omitted to avoid obscuring the
description of the illustrative embodiments.
[0024] STAs 103 by contrast are not necessarily adapted to enter PS
mode. Rather, these devices are adapted to transmit/receive
voice/audio data, or video data, or both. Notably, STAs 103 may be
other than RT STAs. Generally, STAs 103 have comparatively
low-delay requirements, or comparatively low reliability
requirements, or both.
[0025] It is noted that only a few STAs 102, 103 are shown; this is
merely for simplicity of discussion. Clearly, many other STAs may
be used. Finally, it is noted that the STAs 102, 103 are not
necessarily the same. In fact, a plethora of different types of
STAs adapted to function under the chosen protocol may be used
within the network(s) of the system 100.
[0026] In one representative embodiment, the wireless system 100 is
of a type that requires support for polled access for two different
applications that need QoS guarantees, such as a medical telemetry
and monitoring application and Voice over Internet Protocol (VoIP).
Often, medical telemetry and monitoring STAs are PS STAs that also
require comparatively high reliability. Notably, this may be a
common scenario, as VoIP over WLANs are becoming prevalent. For
example, the system 100 is contemplated for use in a hospital that
has to support both medical devices and VoIP, in addition to IT
traffic, over the same WLAN. In this scenario polled access is
expected to play a useful role to guarantee the QoS for the
applications, but the performance of the polled access mechanism
depends on the scheduling method selected. It is emphasized that
the examples provided are merely intended to illustrate one
contemplated implementation of the present teachings. The present
teachings are contemplated for use in a variety of wireless
applications in which STAs of the system have disparate power
consumption, delay and QoS requirements.
[0027] FIG. 2 is a conceptual view of a timing diagram 200 in
accordance with a representative embodiment. The diagram includes a
first beacon interval (BI) 201 and a second BI 202, and each beacon
interval may comprise a superframe or a portion of a
superframe.
[0028] A first beacon transmission period (BT) 203 is initiated at
the beginning of the first BI 201. As will be described in greater
detail herein, in addition to other activity, an AP (e.g., STA 101)
in a centralized network or other STA in a distributed network
performs certain functions relating to polling of STAs, or
scheduling communications between PS STAs 102 and RT STAs 103, or
both, during the ensuing data transfer in the first BI 201, or in
later BIs, or both.
[0029] After the termination of the first BT 203, a PS period 204
begins. The PS period 204 includes PS transmission opportunities
(PS-TXOPs) 206, 207. While only two such opportunities are shown,
the present teachings contemplate a PS-TXOP for each PS STA
scheduled for the PS period 204. The PS-TXOPs 206, 207 allow
respective PS-STAs unfettered access to the medium before access is
granted to other STAs (e.g., RT STAs 103) in the system 100. During
PS-TXOPs 206, 207, the AP 101 (or other STA in a distributed
network) and the respective PS-STAs (e.g., PS-STA 102) may exchange
data or the PS-STAs may exchange data with other STAs in the
network.
[0030] Beneficially, the allocation of PS-TXOPs 206, 207 to
respective PS-STAs 102 after BT 201 reduces the possibility that
the communication start time defined for a PS STA 102 is delayed
due to other transmissions. Notably, the AP 101 has higher priority
of access to the medium. Therefore, the AP 101 can ensure that no
other transmissions take place in the network after the BT 201 and
before the PS-TXOPs 206,207.
[0031] As will be described more fully in connection with FIG. 3,
the PS STAs 102 are polled before any other STA (e.g., an RT STA)
can access the channel after the beacon transmission. Accordingly,
the AP 101 grants the PS-TXOPs 206, 207 at deterministic time
intervals, such that a PS-STA may enter active mode ("wake up") at
deterministic time instants right before the expected polling time,
use the granted PS-TXOP, and then go back into "sleep" (PS) mode.
This operation yields the minimal power consumption for PS-STAs.
Usefully, only the AP 101 (or other STAs that perform the
scheduling) can access the medium after BTs 203, 208. This reduces
the chance of delaying the PS-TXOPs for PS-STAs, thereby enabling
the synchronization between the medium access schedule and PS
mode.
[0032] In a representative embodiment, the PS-STA 102 assigned to
PS-TXOP 206 is a medical telemetry or monitoring device adapted to
transmit patient data to the AP 101, and after completing this
transmission, enters a sleep-mode to conserve power. As will be
appreciated, the data provided by the PS-STA 102 may be vital to
patient care and thus is important. As such, the reliability of the
transmission is important. Moreover, the power conservation
requirement of the PS-STA ensures that monitoring can be completed
over a long period of time without concern of interruption due to
power failure. By virtue of the present scheduling method,
unfettered access provides comparatively high reliability (QoS) and
allows the PS-STA 102 to remain idle until the next PS-TXOP
scheduled at a deterministic time for the particular PS-STA 102,
thus allowing for power conservation.
[0033] After completion of the scheduled PS-TXOPs 206, 207, the PS
Period 204 terminates. After termination of the PS Period 204, the
remaining time in the first BI 201 includes other TXOPs, which are
reserved for non-PS STAs. In a representative embodiment, RT TXOPs
205 are scheduled, with an RT TXOP 205 allocated to a respective RT
STA 103. As noted previously, more or fewer RT TXOPs 205 may be
provided than the number shown. Moreover, other types of STAs may
be allocated TXOPs (not shown) in the remaining portion of the BI
201 after termination of the PS Period 204. The allocation and
media access provided to the RT STAs and other STAs may be in
accordance with a known protocol such as the protocols noted
previously.
[0034] After the termination of the first BI 201, the second BI 202
begins with the commencement of the second BT 208. After the second
BT 208, a second PS Period 209 commences and the exchange of data
continues in a manner described in connection with the first BI
201. In a representative embodiment, PS-TXOPs are granted to the
same PS STAs at deterministic start times in subsequent (e.g.,
consecutive) PS periods. As such a PS-TXOP may be provided to one
PS-STA in PS Period 204 and at a deterministic start time in
PS-TXOP in PS Period 209, and so forth in subsequent BIs.
Alternatively, or additionally, other PS STAs 102 that were not
granted PS-TXOPs in PS Period 204 may be granted PS-TXOPs in PS
Period 209. Also, certain PS STAs 102 that were granted PS-TXOPs in
PS Period 204 may not be granted PS-TXOPs in PS Period 209.
[0035] FIG. 3 is a conceptual timing diagram 300 in accordance with
another representative embodiment. The timing diagram 300 shares
many features in common with the embodiments described in
connection with FIGS. 1 and 2. Duplicative details are omitted to
avoid obscuring the features of the present embodiment.
[0036] The PS Period 204 comprises PS TXOPs 206, 207 as shown. In
certain cases, a PS STA 102 may complete its access to the medium
(e.g., complete its transmission) prior to termination of a
PS-TXOP. After successfully receiving data from the PS STA 102, the
AP 101 (or other STA controlling access to the medium in a
distributed network) transmits an acknowledgement (ACK) to the PS
STA 102. Alternatively or additionally, the AP 101 may transmit
data to the PS STA 102. Upon successfully receiving the data, the
PS STA 102 transmits an ACK to the AP 101. At this point, the PS
STA 102 may enter sleep mode. However, there may be time remaining
in the PS-TXOP that was not used by the PS STA 102. In accordance
with a representative embodiment, at this point the AP 101 (or
other STA having control over the medium) may grant access to the
medium to an STA 103.
[0037] In the timing diagram 300, the PS TXOP 206 includes a used
TXOP 301 and an RT TXOP 302. The used TXOP 301 represents a PS TXOP
that is completed before the termination of allocated time for the
PS TXOP 206. After transmitting or receiving the ACK, the AP grants
the RT TXOP 302 to an STA 103. This STA 103 then accesses the
medium and terminates access by the end of the allocated time of
the RT TXOP 302. Notably, while only one RT TXOP 302 is shown,
there may be more RT TXOPs allocated to other STAs 103 within the
PS TXOP 206, if such allocation is within the bounds of the time
available. After completion of the PS TXOP 206, the medium access
continues as described previously.
[0038] In another representative embodiment, the PS STA 102 may
receive data from the AP 101 (or other STA controlling the access
to the medium) within the PS-TXOP 206. In this case, after the
transmission of the last data frame the AP 102 (or other STA
controlling access to the medium) indicate to the PS STA 102, using
know protocols and methods, that there is no more data to be sent.
After receiving the data, the PS STA 102 responds to the last
transmission with an ACK and if there is remaining time in the
PS-TXOP 26, the AP 101 (or other STA controlling the access to the
medium) may allocate the remaining time in the PS-TXOP 206 to other
non-PS STA, as described previously.
[0039] As will be appreciated, the present teachings allow one or
more STA 103 to access the medium and thus make use of medium
access time that would otherwise go unused. As will be appreciated,
this improves the utilization of the medium. Ultimately, by
allocating more fully medium access time to STAs 103, the overall
reliability and QoS of the system 100 are improved.
[0040] Moreover, and as explained more fully in connection with
FIG. 4, the scheduling of STAs 103 in the remaining time of a
PS-TXOP according to the present teachings will not impact the
start-time of another PS TXOP and thus will not adversely impact
the QoS of PS STAs 102, and other STAs 103.
[0041] In addition to other benefits, the opportunistic scheduling
of representative embodiments can provide hard and soft delay
guarantees for distinct STAs that are classified based on their
power, reliability and delay requirements. As is known, a hard
delay guarantee ensures that the delay bound defined for every STA
admitted to a network will be satisfied, irrespective of the
traffic load in the network. On the other hand, a scheduling
algorithm that provides "soft" guarantees tries to satisfy the
delay bound requirements, but the delay can increase above the
delay bound under overloaded traffic conditions. In representative
embodiments, the PS STAs 102 that are scheduled in PS periods after
beacons can receive hard delay guarantees, while STAs 103 that are
opportunistically scheduled into TXOPs within the PS-TXOPs can
receive soft delay guarantees.
[0042] FIG. 4 is a flow chart of a method 400 of wireless
communication. Notably, certain sub-steps are shown in FIG. 4. The
details of these sub-steps are not described in detail to avoid
obscuring the description of the present illustrative
embodiments.
[0043] As noted, the AP 101 (or other STA if the network is a
distributed network) controls access to the medium.
[0044] According to the governing protocol, the AP 101 receives an
admission request from an STA. Based on information from the
admission request packet data, the requesting STA may be classified
as PS STA 102, RT STA 103 or other type of non-PS STA. For example,
in a representative embodiment, the network functions under the
802.11e MAC protocol. In such an embodiment, the three bits of a
User Priority (UP) field in a Traffic Stream (TS) Info field
contained in the TSPEC is set by STA in the admission request frame
(ADDTS request) to the AP 101.
[0045] The AP 101 may use this information to classify STAs as PS
STAs 103 or RT STAs (e.g., STAs 103). Illustratively, the wireless
protocol of the system may use the UP field to classify the STAs as
PS-STAs 102 or STAs 103. For example, a specific combination of the
three bits could be used to indicate the STA is PS-enabled, and all
other combinations could be classified as RT STAs.
[0046] Once the request is received, the AP 101 executes an
admission control procedure to decide whether to admit this new
STA. In a representative embodiment, if the STA is admitted, the AP
101 creates one entry for the STA in one of the following two
lists: [0047] A fixed scheduling list (FS_list) with one element
per PS STAs admitted by the AP; and [0048] A list with RT STAs
(RT_list), i.e., STAs admitted to use the polling based access but
that are not PS STAs.
[0049] In another representative embodiment, each element X of the
FS_list contains at least the following attributes: [0050]
X.Address=MAC address of the STA; [0051] X.start_time=The expected
start time of the next TXOP for this STA; [0052] X.SI=the Service
Interval between consecutive service periods; [0053]
X.TXOP_Duration=The amount of time to granted to the STA in the
next service period.
[0054] The FS_list could be ordered according to the start_time
attribute and the start times of two consecutive elements in the
FS_list, X.sub.n and X.sub.n+1, should satisfy the following
condition:
X.sub.n+1.start_time>X.sub.n.start_time+X.sub.n.TXOP_Duration.
(1)
[0055] This condition avoids overlapping of the TXOP granted to PS
STAs, thereby ensuring that a PS STA 102 will not be awake, while
waiting for the end of a ongoing TXOP before it can be served.
[0056] In another embodiment, the AP 101 assigns the service
interval (SI) for all PS STAs 102 as a multiple of the Beacon
Interval (B_interval) given by:
X.sub.n.SI=.beta.*B_interval, for .A-inverted.n, where .beta.=1,2,
. . . (2)
[0057] In an embodiment, the integer constant .beta. could be
defined as:
.beta. = Req_SI B_interval , ( 3 ) ##EQU00001##
[0058] where Req_SI denotes the SI requested by the PS STAs 102 in
the TSPEC and .left brkt-top.x.right brkt-bot. denotes the largest
integer smaller than x. In order to efficiently utilize the PS mode
to save power, the STA must generate a deterministic traffic
pattern and most likely it will have a constant SI, otherwise it
would be impossible to achieve a synchronization with the
scheduler.
[0059] A next beacon transmission (e.g., in BT 208) by the AP 101
at any time instant is given by next_TBTT and req_start_time(A) is
the expected start time indicated by a given STA to the AP in the
admission request frame (ADDTS Request). The illustrative method
described in connection with FIG. 4 is useful to insert the PS STA
into the FS_list. The main idea is to grant the first poll in the
first available position after the first beacon after the
req_start_time.
[0060] At step 401 the zeroth order interval begins. At step 402,
the AP 101 first determines after which future beacon the STA could
be served for the first time based on the requested start time. In
particular, the AP 101 determines if the requested start time is
greater than the next time target beacon transmission time or if it
is greater than the target beacon transmission time of a future
beacon. Once the future beacon after which the STA could be served
is selected, the AP 101 checks in step 403 the current number of PS
STAs in the FS_list. If the FS_list is empty, then, at step 404,
the AP 101 inserts the new STA in the first position of the list
and sets the actual start time for the PS-TXOP for an interframe
space interval (e.g. PIFS in the 802.11 standard) after the
transmission of the future beacon frame. At step 405, in case there
are other PS STAs in the FS_list, the AP 101 determines whether the
new STA should be polled after the next PS-TXOP granted to the last
PS STA in the FS_list, or a in service interval thereafter.
[0061] Next, in step 406 if the start time of the last STA in the
FS_list (Xn.start_time) is earlier than the start time of the BI at
which the new STA should be polled the AP 101 inserts the new STA
in the end of the FS_list and sets its start time for a service
interval after the next PS-TXOP granted to the previously last STA
in the FS_list. Otherwise, the AP 101 in step 407 inserts the new
STA in the last position of the FS_list and is polled after the
next poll for the previously last STA in the FS_list.
[0062] The method of FIG. 4 is useful to insert new PS STAs in the
FS_list and to set the time of their first PS-TXOP, which
considering all other STAs already in the FS_list. This ensures
that the STA is not polled before the requested start time.
Furthermore, the AP 101 polls STAs from the FS_list at the
corresponding start times and every time a STA in the position n is
polled, the AP 101 updates its start time a follows:
X.sub.n.start_time=X.sub.n.start_time+X.sub.n.SI. (4)
[0063] Due to the proposed system and method and the fact that all
PS STAs are assigned the same SI, the scheduling order does not
change as the STAs are polled. The only difference from one beacon
to another are new STAs that may be added at the end of the
list.
[0064] In one embodiment, when a PS STA is inserted into the fixed
scheduling list, the AP 101 (or other STA controlling the access to
the channel) could define the TXOP_Duration granted to the STA as
the amount of time need to allow the STA to transmit all data
frames generated since the last TXOP plus all possible
retransmissions allowed by the protocol for each data frame.
[0065] In another embodiment, the extra-time for retransmissions
added to the TXOP, could be dependent on the channel conditions,
which can be inferred through channel measurements. The better the
channel conditions, the less extra-time would be granted per
TXOP.
[0066] As noted previously, the opportunistic scheduling may be
used to further utilize the medium access. FIG. 5 is a flow chart
of a method 500 of wireless communication that provides for
opportunistic scheduling. Notably, certain sub-steps are shown in
FIG. 5. The details of these sub-steps are not described in detail
to avoid obscuring the description of the present illustrative
embodiments.
[0067] When a RT STA is admitted, a new entry is included in the
RT_list. In one embodiment, each element Y in the RT_list would
contain, at least, the following attributes: [0068] Y.Address: MAC
address of the STA; [0069] Y.mSI: minimum time interval between two
consecutive service periods; [0070] Y.MSI: maximum time interval
between two consecutive service periods; [0071] Y.DB: maximum delay
supported by STA [0072] Y.TXOP_Duration: amount of time to granted
to the STA in the next service period. [0073] Y.last_TXOP_time: the
time of the last TXOP was granted to this STA.
[0074] At step 501 the method waits for one of the both events,
namely: a PS TXOP is terminated early or the end of a PS period. At
step 503 of the method determines if the PS STA has terminated
transmission before the termination of a PS TXOP. This would occur
for example, if an ACK is transmitted by the AP and the polled STA
has no more data to transmit within the allocated PX TXOP. If so,
at step 505 the AP (i.e., the scheduler) checks a second list,
SC_RT_list, which contains one entry for each STA that is eligible,
at a given time instant, to receive a TXOP from the AP, to decide
which STA to grant the a TXOP using the remaining time in the
terminated PS TXOP. The SC_RT_list is updated as the first task in
step 505. In an embodiment, the SC_RT_list is updated at a given
time instant T by the AP 101 searching the RT_list for elements
that satisfy the following condition:
Y.mSI.ltoreq.T-Y.last.sub.--TXOP_time.ltoreq.Y.MSI. (5)
[0075] In one embodiment each element in the SC_RT_list would be a
copy of the element in the RT_list with one additional attribute to
define the deadline for the corresponding STA, which would be
defined as
Y.D=Y.last.sub.--TXOP_time+Y.DB. (6)
[0076] Furthermore, the SC_RT_list is ordered by the early dead
line first policy. In the rest of step 505 the AP 101 selects the
STA from the SC_RT_list to be granted the remaining TXOP time.
[0077] At step 502 the method checks if the PS period has ended.
This provides an opportunity for non-PS STAs to be granted access
to the channel. If this is the case, at step 504 the AP 101 updates
the SC_RT_list according the earliest deadline as described
previously. If the STA at the beginning/top of the list is eligible
to receive a TXOP in the current beacon interval, the AP 101
removes the STA from the SC_RT_list, schedule a TXOP to the STA and
repeats the same process for other STAs in the SC_RT_list.
[0078] In one embodiment, the opportunistic scheduling algorithm of
FIG. 5 could be used by the AP 101 to grant TXOPs to RT STAs. As
can be noted, the RT STAs are scheduled in two different
situations: 1) after all PS-enabled STAs have been scheduled in a
beacon period (and the method continues at step 504); or 2) when a
PS-STA does not use its complete TXOP and the remaining time is
enough to allocate a RT TXOP (and the method continues at 505).
Another important feature in the algorithm is that RT STAs are
scheduled according to their deadlines.
[0079] In the representative embodiments described herein, a
wireless method and system provide priority access to STAs having
PS requirements. As will be appreciated by one of ordinary skill in
the art, many variations that are in accordance with the present
teachings are possible and remain within the scope of the appended
claims. These and other variations would become clear to one of
ordinary skill in the art after inspection of the specification,
drawings and claims herein. The invention therefore is not to be
restricted except within the spirit and scope of the appended
claims.
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